Collecting and monitoring data from a variety of locations for input into a central processing unit is a required function in a wide variety of applications. For example, in a chemical processing plant, such as a refinery, a variety of process parameters such as temperatures, pressures and flows are continuously or intermittently monitored and recorded. Similarly, in oil and gas production parameters such as pressures, flows and valve positions are measured and monitored. Likewise, certain types of environmental Typically, these parameters are measured at different locations in the particular process, and in some cases the instrumentation may be located at a remote site. Measurements are normally collected by the instrumentation as analog signal that is converted to a digital format and subsequently transmitted to a computing device such as a programmable process controller or process control computer via dedicated hard wired serial data interfaces. However, in is the case where the instrumentation is located at a remote site, it may be difficult or cost prohibitive to provide the direct hard wired connection between the serial data interface at the particular instrument's location and the interface at the particular computing device. On the other hand, wireless transmission of digital data over long distances has heretofore proven unreliable.
Cellular Digital Packet Data (“CDPD”) is a wireless communications protocol that folds streams of data into envelopes or packets that are transmitted at very high speeds during pauses in cellular phone conversations. This permits the use of existing cellular systems as a means of data transmission. CDPD allows data files to be assembled into packets and transmitted via idle channels of existing, but CDPD-upgraded, cellular voice networks. CDPD allows the transmission of data at 19.2 Kbps over an enhanced cellular network. Adding CDPD to an existing analog cellular system allows cellular systems to transmit data eight times faster without the necessity of creating a completely new digital system.
In practice, packet data is transmitted in a wireless mode using bandwidth of the Advanced Mobile Phone Service (“AMPS”) which operates as the communications infrastructure for analog cellular radio. Digital cellular is referred to as D-AMPS. CDPD specifications are published through the CDPD Forum and follow OSI (Open Systems Interconnection Protocol) guidelines. CDPD technology provides connectivity up to the network layer and is an overlay system that operates on AMPS frequencies.
The RS232 serial interface provides a serial data connection between two devices over dedicated wires. The interface defines up to 26 lines between two devices. One line carries the data and the others carry signaling information. Signaling is achieved by the lines through binary states, either “ON” or “OFF.” Some lines are defined for data and some for signaling. Thus, data transmission can be controlled simultaneous by both the sending device and the receiving device.
For example, the sending device can query the receiving device as to whether it is ready to receive data by setting the signal high on a line called Ready To Send (“RTS”). The receiving device can, in turn, reply that it is ready to receive data by setting the signal high on the Clear To Send (“CTS”) line. After these conditions are satisfied, by devices can begin data transmission over the data lines a communication speeds.